1. Field of the Invention
The present invention relates to the enhanced production of latex rubber from the guayule plant, a desert shrub that is native to northern Mexico and the southwestern United States.
2. Description of the Art
World-wide consumption of natural rubber was 6.6 million metric tons in 1998, with the US importing over $10 billion worth of raw material and finished rubber goods. Currently the only commercially harvestable source of natural rubber is a tropical plant, Hevea brasiliensis. Because the genetic stock of this crop plant is very narrow and the growing range of the plant is limited to the tropics, other sources of natural rubber have been sought during the last century. One such plant that has been studied extensively during this time is Parthenium argentatum Gray (guayule).
Guayule is a perennial, indigenous to the Chihuahuan desert of northern Mexico (Lloyd, 1911) and southwestern U.S., that produces rubber during the late fall and winter months (Cornish and Siler, 1996). It is currently being developed as a new industrial crop because it promises to be the source of high quality natural rubber which also happens to be hypoallergenic (Carey et al. 317-20; Siler, Cornish, and Hamilton 895-902). Current breeding efforts to increase rubber yield in this plant have raised the level of rubber yield to about 10% dry weight (Estilai and Ray, in Whitworth and Whitehead). The biochemical pathways of the plant have also been the subject of extensive investigation. (Bonner and Arreguin (1949), Benedict (1983), Cornish (1993), Cornish and Siler (1995), Castillón and Cornish (1999)).
The synthesis of natural rubber involves the polymerization of thousands of molecules of isopentenyl diphosphate added to an allylic diphosphate initiator (usually assumed to be farnesyl diphosphate). Rubber biosynthesis represents an offshoot of terpenoid metabolism in the plants that produce it and occurs outside the plastid on cytoplasmic vesicles contained by a lipid monolayer (Cornish, Wood, and Windle 85-96). The formation of this secondary metabolite occurs within a symplastic laticifer in Hevea brasiliensis and within the bark parenchyma cytoplasm of P. argentatum (Backhaus 283-93). The absence of laticifers in the rubber synthesis in P. argentatum makes this plant a simpler model than H. brasiliensis to study the regulation of natural rubber production in vivo. In addition, P. argentatum produces a resin rich in sesquiterpene- and triterpene-derived components (Schloman, Jr. et al. 873-76), which may possess promising industrial uses and whose content might be influenced by manipulating the terpenoid pathway.
Manipulating the levels of substrates that can be used by the rubber transferase in the terpenoid biosynthetic pathway, therefore, should result in increased yield or improved quality of latex rubber. One such target in the metabolic pathway involves the allylic diphosphate initiators.
Manipulating the levels of substrates that can be used by the rubber transferase also should result in increased yield of the resin components produced by the terpenoid biosynthetic pathway.